Gas damped deceleration switches that activate an airbag inflator in a vehicle in response to vehicle deceleration are known. One such gas damped deceleration switch is shown in copending U.S. Pat. application Ser. No. 664,497, filed Mar. 5, 1991, and assigned to the present assignee, which is a continuation of U.S. Pat. application Ser. No. 491,450, filed Mar. 9, 1990, now abandoned. The gas damped deceleration switch shown in the co-pending application is an electrical switch comprising a mass supported for movement in response to vehicle deceleration. The mass is spring biased into a rest position, and is movable against the bias of the spring toward an electrical contact. When moved to the electrical contact by deceleration of the vehicle, the mass and the electrical contact complete an electrical circuit to energize an airbag inflator.
The deceleration switch disclosed in the co-pending application further comprises a movable damping member which is connected to the mass for movement with the mass, and a stationary base structure defining a cavity. When the mass is in the rest position, the movable damping member is held in engagement with the base structure to define a closed volume within the cavity between the base structure and the damping member. As the damping member is carried by the mass away from the base structure, the closed volume between the base structure and the damping member is enlarged. Enlargement of the closed volume creates a pressure reduction within the closed volume, and thus creates a relative vacuum within the closed volume. The vacuum results in a pressure differential acting across the moving damping member. This pressure differential results in a damping force acting against the moving damping member. The damping force resists movement of the mass toward the electrical contact.
If deceleration of the vehicle is of sufficient magnitude and duration, the mass will be moved against the damping force, as well as against the bias of the spring, to carry the damping member away from the stationary base structure and to open the closed volume between the damping member and the base structure. Thus, the vacuum will no longer exist. Further movement of the mass and the damping member is resisted by the continuing bias of the spring, and by a minimal amount of damping force as required to displace the gas around the moving damping member. If deceleration of the vehicle is not of sufficient magnitude and duration to cause the moving mass to overcome the damping forces, the moving mass and damping member will be moved back into their rest positions by the bias of the spring.
The deceleration switch disclosed in the co-pending application includes a passage that enables a flow of gas to be directed into the closed volume in response to the relative vacuum created in the closed volume, and a valve for adjusting the size of a gas flow space in the passage. The valve comprises a cap having threads engaged with threads on the base structure. Rotation of the cap in one direction enlarges the gas flow space, and rotation of the cap in the other direction reduces the gas flow space. For a given rate of movement of the mass, the rate at which a vacuum is generated, and consequently the degree to which the vacuum will restrain movement of the mass, is increased by decreasing the size of the gas flow space. For the same rate of movement of the mass, the rate at which a vacuum is generated, and consequently the degree to which the vacuum will restrain movement of the mass, is decreased by increasing the size of the gas flow space. The valve thus regulates the degree to which the vacuum will restrain movement of the mass. The deceleration switch can thus be calibrated by adjusting the valve.